Weakly Buffered Soils

When adding lime to acidic soils to increase the soil pH, the lime needs to neutralize the acidity in the soil solution along with the acidity released from the soil’s CEC in the process. There is a pool of reserve of acidity on the soil CEC that is released to maintain equilibrium between the CEC and soil solution. The impact of this reserve acidity held by the cation exchange sites will vary depending on the CEC of the soil.

The rate of lime to apply in the goal of neutralizing acidity is determined by using the buffer pH for soils with a CEC above 7, and organic matter below 20%. The buffer pH is a measurement of all acidity in the soil solution along with the reserve acidity held by the CEC of the soil. As soil CEC decreases so does the pool of reserve acidity. Below a CEC of 7 the reserve acidity is small and has little impact on lowering the buffer pH. Therefore, buffer pH is not a good indicator of lime application rate for these soils.

For weakly buffered soils, lime is applied at 1 ton/acre for each 0.3 - 0.4 of desired increase in soil pH, maximum 2 ton/acre. pH adjustments are usually triggered when the soil pH is 0.3 units lower than the target pH for the field.

A weakly buffered soil can have a CEC in high 6’s to low 7’s. To determine if a soil is weakly buffered, calculate lime rate for soils with a water pH of 6.2 or below using the weakly buffered rate above and an equation using buffer pH, use the higher of the two calculated lime rates. For more information on weakly buffered soil, contact you regional ALGL sales agronomist.